In engine systems with split exhaust manifold, a blowdown exhaust valve of a cylinder may be opened first to deliver exhaust mass flow from an initial portion of an exhaust phase to a turbine of a turbocharger or a turbine-driven generator, while a scavenging valve may be opened later to deliver exhaust mass flow from a latter portion of the exhaust phase directly to an exhaust catalyst, bypassing the turbine. In this way, by directing exhaust gases away from the turbine during the latter portion of the exhaust phase, pumping penalty associated with high turbine backpressure may be reduced.
One example of such a split exhaust engine system is illustrated by Robel in U.S. Pat. No. 8,091,357. Therein, an exhaust system includes a turbo compounding device located in a first exhaust branch, and an exhaust gas treatment device is located in a second exhaust branch. Further, a valve is interposed between the first exhaust branch and the second exhaust branch, which when opened directs exhaust gases away from the turbo compounding device to the exhaust gas treatment device.
However, the inventors herein have recognized potential issues with such a system. As one example, during some operating conditions, it may be desirable to bypass exhaust gas flow around the turbine in order to limit a maximum speed for the turbine-generator; limit an output of the generator; and/or limit a rate of change of a speed of the generator. Further, during cold-start conditions, it may be desirable to bypass the turbine in order to direct a majority of exhaust energy to the exhaust catalyst to expedite catalyst warm-up. Utilizing a valve to bypass the turbine necessitates need for an additional actuator. Consequently, production cost and packaging space may increase resulting in bulky and expensive exhaust systems.
Thus in one example, some of these issues may be at least partly addressed by a method for an engine, comprising: delivering exhaust from a first exhaust valve of a cylinder to an exhaust turbine via a first exhaust manifold, the turbine driving a generator; delivering exhaust from a second exhaust valve of the cylinder to an exhaust catalyst while bypassing the turbine via a second exhaust manifold; and selectively deactivating the first exhaust valve in response to a turbine speed greater than a threshold turbine speed. In this way, by utilizing valve deactivation, an amount of exhaust mass flow to the turbine may be reduced.
For example, a split exhaust engine system may include a first exhaust valve (herein referred to as blowdown valve) for delivering a first portion of exhaust energy (herein referred to as blowdown energy) to a turbine of a turbine-generator located in a first exhaust passage. The engine system may further include a second exhaust valve (herein referred to as scavenging valve) for delivering a latter portion of exhaust energy (herein referred to as scavenging energy) to an exhaust catalyst located in a second, different exhaust passage. During engine operating conditions when a turbine speed of the turbine-generator is greater than a threshold speed or when a generator output is greater than a desired output, an engine controller may reduce an amount of exhaust gas (that is, blowdown gas) delivered to the turbine by deactivating the blowdown valves in a number of cylinders in the engine system. The number of blowdown valves that may be deactivated may be based on one or more of a rate of change of turbine speed, a difference between an actual generator output and a desired generator output greater than a threshold difference, and an engine speed/load condition. In some examples, the blowdown valves on all cylinders of the engine may be deactivated to limit exhaust flow to the turbine.
In one example, during cold start conditions, exhaust energy to the turbine may be limited via blowdown valve deactivation in order to direct a majority of exhaust gases to an exhaust catalyst located downstream of the turbine-generator for faster catalyst warm-up.
In this way, one or more blowdown valves may be deactivated to reduce the amount of exhaust energy delivered to the turbine to limit the maximum speed of the turbine-generator system, limit the output of the generator, limit the rate of change of the turbine-driven generator speed, and/or to expedite catalyst warm-up. By utilizing valve deactivation, a wastegate valve for controlling exhaust flow to the turbine may be avoided. Further, by utilizing valve deactivation instead of the wastegate, a bulky, expensive actuator for actuating the wastegate may be avoided. Consequently, system cost and packaging may be reduced. Still further, by avoiding the wastegate, a volume upstream of the turbine may be reduced. Consequently, utilization of blowdown energy may be improved, thereby increasing the turbine-generator efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.